Denials

A crowd of onlookers and journalists had gathered, and shouts of "thief!" and "where is my envelope?" could be heard.

On Saturday, Mr Rajoy went on TV to deny ever having received secret or improper payments.

Rajoy scandal - the key questions

What are the allegations? That documents published by El Pais are a list of undeclared or secret cash payments to senior PP members

Is Spain's prime minister involved? Mariano Rajoy's name is written a number of times in the documents.

How damaging could this case be? The scandal has already damaged the image of Mr Rajoy and his party because it has received such widespread media coverage.

What happens next? Spain's chief state prosecutor says there could be enough evidence to investigate. Mr Rajoy says he will publish his earnings online. Opposition leaders have called for him to resign, but as things stand, that looks unlikely.

Small groups of demonstrators took to the streets in Madrid, Barcelona and Seville calling on him to step down, and clashes with police took place in the capital at the weekend.

Mr Barcenas stepped down as treasurer in 2009 after being implicated in a separate corruption case known as the Gurtel scandal.

In that case, he stands accused of tax fraud and receiving illegal payments, claims he also denies.

He is due to be questioned by investigating judge Pablo Ruz about the Gurtel case on 25 February, El Pais reports.

Correspondents say the latest allegations have particularly angered the public because Mr Rajoy's government is demanding huge sacrifices from the public as the country battles recession and 25% unemployment.

The central claim is that documents published by El Pais are a list of undeclared cash "donations" linked to senior PP members.

Until 2007, Spanish political parties were allowed to receive anonymous donations.

Spain's chief prosecutor has said there could be enough evidence to investigate the allegations.

Political damage

The PP has said it will take legal action against those responsible for what it says is a smear campaign, and Mr Rajoy has promised to publish his earnings online.

He [Rajoy] is not going to be listened to the slightest bit by the European institutionsFerran Requejo, Political scientist at Barcelona's Pompeu Fabra University

Opposition leader Alfredo Perez Rubalcaba has called for him to resign, but the BBC's Tom Burridge in Spain says that, as things stand, that looks unlikely.

Political scientist Anton Losada of Santiago de Compostela University said that the prime minister would find it harder to implement austerity policies.

"There is a high political cost to pay, with a problem of legitimacy," he told AFP news agency.

"He cannot demand sacrifices and at the same time make money, especially if it is not legal."

According to Ferran Requejo, a political scientist at Barcelona's Pompeu Fabra University, Mr Rajoy would now struggle to be taken seriously in the eurozone.

"An accusation of corruption in a governing party is a serious accusation," he told AFP news agency.

"He is not going to be listened to the slightest bit by the European institutions."